1. Technical Field
The present disclosure relates to a method and a device for characterizing or measuring a capacitance. The present also relates to a method and a device for characterizing or measuring a floating capacitance.
The present disclosure relates in particular but not exclusively to the characterization or measurement of capacitances in a touch screen display.
2. Description of the Related Art
Various methods for characterizing or measuring capacitances are known, such as charge transfer methods, time constant measurement methods, oscillator relaxation methods, charging current measurement methods, methods using a sigma-delta modulator and sine wave measurement methods. These methods will be briefly summed up.
Charge Transfer
Charges are transferred from a precharged reference capacitance to a capacitance to be measured. The operation is repeated several times with calibrated charge quantities until reaching a determined voltage. The number of charge transfers to obtain this voltage is representative of the capacitance to be measured.
Time Constant Measurement
A resistor of known value R is connected in series with the capacitance C to be measured. A switch is put in parallel with the capacitance. The capacitance is charged and is then discharged using the switch. An amplifier copies the decrease curve of the voltage at the terminals of the capacitance. The time constant RC corresponding to the decrease curve of the voltage is determined, to deduce the capacitance C therefrom.
Oscillator Relaxation
A current generator is put in series with the capacitance to be measured. A transistor is put in parallel with the capacitance. The voltage at the terminals of the capacitance is sent to the positive input of an operational amplifier which negative input receives a reference voltage. The output of the amplifier controls the transistor. The set forms an oscillator which frequency F is a function of the capacitance C. Measuring the frequency allows the capacitance to be determined.
Measurement of a Charge Current
A constant charge current I is injected into the capacitance C to be measured. After a determined time T, a charge Q is accumulated by the capacitance C and the voltage at the terminals of the capacitance goes from an initial voltage Va to a voltage Vb. By applying the relationships Q=C*V and Q=I*T, the capacitance C which is equal to I*T/(Vb−Va) is deduced.
Measurement with a Sigma-Delta Modulator
An alternating voltage is applied to the terminals of the capacitance C to be measured. The voltage at the terminals of the capacitance is digitized by a sigma-delta modulator and the value thereof is representative of the capacitance.
Sine Wave Measurement
A resistor R is put in series with the capacitance C to be measured. An alternating signal is applied to the set. The voltage at the terminals of the resistor is measured with a differential analog-to-digital converter, to deduce the capacitance C.
Some known methods are penalized by a quite long acquisition time of the parameter allowing a capacitance to be characterized or measured. Others are penalized by the complexity of the means used for their implementation. Applications such as the control of a touch screen display require a rapid scan time of a matrix of capacitances, to determine the variations of its capacitances and detect the actions of the user.